1. Field of the Invention
This invention relates to lubricating oil compositions having a low foaming tendency and a low concentration of silicon, and more particularly to the use of estolides as antifoaming agents.
2. The Prior Art
Oil based lubricants, hydraulic fluids, and the like, which are subject to stress during use contain additives similar to detergents which are used to prevent sludging, formation of gums, lacquer, resinous and other similar materials, on the surfaces of the devices which are lubricated or contacted by the oil. Deposition of such materials can lower the operating efficiency of the device using the lubricant. However, these detergent-like additives tend to cause the oil to foam during use. Such foaming, or air entrapment, is undesirable because the air occupies space between the surfaces to be lubricated thereby preventing proper lubrication of the surfaces by the oil. Further, the entrapment of air in the oil can cause an increase in the rate of oxidation. Such oxidation tends to form unwanted contaminants which further reduce the lubricating ability of the oil.
In many oils, foaming was controlled by the addition of a small quantity of a silicone fluid. The useful silicone fluids include silicone polymers, such as dimethyl silicones. However, many manufacturers of internal combustion engines and other devices which employ lubricating oils, not only specify the maximum amount of foaming permitted for the lubricating oil, but also specify the maximum concentration of silicon in the lubricating oil. Such silicon specifications arise from the fact that lubricating oils are regularly tested to determine the condition of the device using the oil. Since silicones are used as water conditioners in the cooling systems of many devices, the presence of relatively large quantities of silicon in the oil would give a false indication of a cooling system leak into the oil or would make such testing useless.
Presently preferred detergent-like additives, such as normal and overbased sulfurized calcium alkylphenolates disclosed in coassigned U.S. Pat. Nos. 4,169,799; 3,761,414; 3,549,534; and 3,474,035, and in coassigned U.S. patent application Ser. No. 78,346, filed Sept. 24, 1979, cause the lubricating oils to which these detergents are added to foam excessively during use. The quantity of the previously used silicone antifoaming agents required to reduce foaming to an acceptable level was found to be much higher than that allowed by many manufacturers of the devices using the lubricating oils.
Coassigned U.S. Pat. No. 3,909,425 discloses the use of an estolide of 12-hydroxystearic acid in a lubricating oil composition which contains many other ingredients, several of which are disclosed as being essential. None of the disclosed essential ingredients are used in the lubricating oil of the present invention. One of the ingredients disclosed as being essential is a zinc compound which would not be used in a lubricating oil composition intended for railway diesel engines because of the detrimental effect of zinc compounds on the silver components of the railway diesel engine. Further there is no disclosure of the use of the estolide of 12-hydroxystearic acid as an anti-foaming agent.
Coassigned U.S. Pat. No. 3,429,820 discloses a method for operating a power steering unit using a lubricating oil containing an estolide of 12-hydroxystearic acid. The estolide of 12-hydroxystearic acid is disclosed as imparting improved antisqueal properties to the power steering fluid. There is no disclosure that the estolide of 12-hydroxystearic acid has any antifoaming properties.
Coassigned U.S. Pat. No. 2,877,181 discloses the use of estolides, including 12-hydroxystearic acid, as stabilizers for anhydrous calcium fatty acid greases. There is no disclosure that the estolide of 12-hydroxystearic acid has any antifoaming properties.